Compact heat exchangers with the hydraulic diameter of the channels in the mm range dominate the high tech industry and are gradually winning the majority of the consumer market due to their high heat transfer area-to-volume and heat transfer area-to-mass ratios. Following the requirements of the market, manufacturers of the compact heat exchangers have sought to reduce the production costs of such heat exchangers to make them comparable in cost to conventional shell-and tube or tube-and-fin type heat exchangers.
New developments in microfabrication and in fluid cleaning, stimulated by electronic development, have made possible the fabrication of heat exchangers with micron sized channels. Such technological advancement potentially allows fabrication of heat exchangers with significantly increased (by orders of magnitude) heat transfer area-to-volume and- mass ratios.
Microchannel techniques have demonstrated superior performance in heat transfer over other technologies. Their compactness and extremely high heat transfer area-to-volume ratios make them a desirable component of heat exchanging systems. Typical heat transfer area-to-volume ratios are higher than those of other geometries for heat exchangers or sinks used in electronics. The fluid handling techniques developed in electronics cooling have made the application of microchannels in heat sinks and heat exchangers possible and desirable for many industries.
New developments in microfabrication technology have also made the fabrication of microchannels relatively inexpensive. A microdeformation technology marketed by Wolverine Tube, Inc., for example, offers the fabrication of microchannels with dimensions up to 2-3 micrometers in width in a wide range of materials, thus making microchannels affordable for a wide range of applications. However, attempts to combine new technology with conventional design thus far have resulted in heat exchangers which are either too expensive or characterized by unsatisfactory performance parameters. Particularly, in many such microfabricated heat exchangers are found lacking in pressure drop and pumping power characteristics.
The application of microfabricated surfaces for manufacturing heat pipes and capillary loops serves to optimize the operative structure of heat pipes. Known heat pipes and capillary pump loops are disadvantageously two-dimensional. Most are fabricated on a single wafer with a cover, where the liquid and wafer channels as well as the capillary structure are located on the same plane. Capillary channels extend along the same plane in continuous length, thus causing a substantial pressure drop as liquid passes therethrough. This tends to limit the capacity of the capillary loops.
Known types of two-phase heat transfer device operation include, pool boiling, convection boiling, and capillary loop operation. Pool boiling relies mostly on surface tension in supplying liquid to a super heated boiling surface. A narrow boiling channel provides higher surface tension and promotes operation at higher heat flux. However, a higher pressure drop imposed on liquid flow in the given channels limits liquid supply. This makes it practically impossible to provide an optimum relationship between the channel's hydraulic diameter and length, given conventional arrangements for the pool boiling surface.
Convection boiling operation usually occurs in channels of substantially constant geometry. An external force (generated by a pump, for instance) is applied to the fluid in order to move it through the channels. Both phases of the fluid (the cooling medium), e.g. liquid and vapor, move along the channels together. The lengths of the channels are typically several times greater than their hydraulic diameters. With smaller channels (in sub-millimeter range, for example) which tend to provide the most efficient heat transfer, the required external force as well as the required fluid circulation pumping power may be quite high in magnitude.
Capillary loop operation is used in heat pipes and capillary pump loops. Similar to pool boiling, capillary loop operation relies also on capillary forces to supply liquid to the operative surface. The liquid transport distance is typically higher than in pool boiling; therefore, greater capillary force and smaller capillary dimensions are required. Also similar to a pool boiling, the amount of the supplied cooling medium is limited by pressure drop in the capillary structure. Conventional technologies for fabrication of heat pipes and capillary pump loops do not permit the optimization of the relationship between a capillary's hydraulic diameter and length.
There remains a need for a lightweight compact microchannel based heat exchangers having high heat transfer performance with low pressure drop and low manufacturing costs.